A known thermal-type fusing apparatus used in an image forming apparatus has an energized heating element serving as a heat source, a power supply for supplying current to the heating element, temperature detecting means for detecting temperature in the vicinity of the heating element, and control means for controlling the supply of current from the power supply based upon a signal from the temperature detecting means. A printing medium carrying an unfixed image formed thereon by an image forming section of the image forming apparatus is heated and fused onto the medium. This arrangement regulates the image fusing temperature to a prescribed temperature for fixing the image.
In such a heat fusing apparatus, the fusing function will not be implemented if even one of the above-mentioned components, namely the heating element, power source, temperature detecting means or control means, does not function normally. Furthermore, there is the danger that the apparatus will be destroyed by overheating when energization runaway occurs. Accordingly, a fusing apparatus of this kind is equipped with the following abnormal-overheat safety devices to thereby prevent overheating, emission of smoke and fire at the time of energization runaway, as described by way of example in the specification of Japanese Patent Application Laid-Open No. 08-248813:    (1) By inserting a safety device (thermo-protector) such as a temperature fuse or thermostat into the energizing circuit of the heating element, passage of current into the heating element is interrupted when overheating occurs owing to energization runaway.    (2) In a case where temperature detecting means such as a thermister is placed in the vicinity of the heating element and the heating element becomes abnormally overheated, passage of current into the heating element is interrupted by current cut-off means such as a relay inserted into the energizing circuit. By setting the temperature at which the abnormal-overheat safety device operates to a temperature higher than that reached at the time of normal operation, malfunction at the time of normal operation can be prevented and the device will operate only at the time of abnormal overheating.
Further, there are abnormality sensing devices in which the output of a thermister is compared with a reference voltage and the heater forcibly opened. Specifically, there is a device (Japanese Patent Application Laid-Open No. 03-034005) in which the reference voltage used for the comparison is changed over for a fixed period of time during which a heater on/off signal is in the off state, a device (Japanese Patent Application Laid-Open No. 03-062207) in which a reference temperature for comparison used in sensing the abnormality is changed in dependence upon a change in control temperature between temperature during standby and temperature at the time of fusing, and a device (Japanese Patent Application Laid-Open No. 08-262923) in which a reference temperature for comparison used in sensing the abnormality is lowered once the reference temperature has been exceeded.
With regard to handling after the above-described abnormal-overheat safety device has been actuated, a serviceman need only replace the failed component of the fusing apparatus or the unit (heating element, power supply, temperature detecting means, control means, etc.) and the spent safety device such as a thermostat that has been actuated.
In actuality, however, owing to an excessive rise in environmental temperature from onset of overheating trouble in the fusing apparatus to actuation of the safety device such as a thermostat, not only such members as a pressure roller and ceramic heaters inside the fusing apparatus but also devices peripheral thereto are subjected to damage such as deformation and degradation. In a worst-case scenario, the entire fusing apparatus and peripheral devices must be repaired or replaced. In order to reduce damage that occurs at the time of energization runaway, the operating temperature of the abnormal-overheat safety device should be set very low. If the operating temperature is set very low, however, the abnormal-overheat safety device will be actuated at the time of normal operation and the image forming apparatus will malfunction.